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Still scary The author of a paper on a mutant bird flu strain says experts agreed to publish it only after he explained that the virus was "much less lethal" than previously feared.

A panel of US science and security experts last week said two papers on mutant viruses should be published after all, reversing its earlier decision to withhold key details.

Professor Ron Fouchier of the Erasmus Medical Centre in Rotterdam, author of one of the papers, told journalists in London that his revised version addressed fears that the paper's findings could be used by bioterrorists.

The announcement came after the revisions to the papers were reviewed by the nongovernmental US National Science Advisory Board for Biosecurity (NSABB).

The US experts had previously opposed publishing the research, which showed how an engineered H5N1 flu virus could pass easily in the air between ferrets, over fears it could end up in the wrong hands and result in a deadly human-induced flu pandemic.

Fouchier says his revised version makes clear that the mutant virus is "much less lethal" than the NSABB had previously believed.

"I did say that it's one of the most dangerous viruses, and it's the truth, because these viruses are a little scary," says Fouchier.

"If they go airborne they can cause pandemics, and pandemic flu has killed millions of people."

Some members of the advisory board understood that the ferrets in the experiment had all died as a result of being infected, leading to the paper being blocked.

"The information was in the original paper but perhaps it was not as clear as it should have been," says Fouchier.

"Our virus does not kill ferrets when it is in aerosol. This was in the original manuscript but it was not spelt out."

The NSABB faced criticism after it ruled unanimously in December that a pair of US-funded studies, one by a team from Wisconsin and the other led by Fouchier, should not be printed without heavy edits of key details.

Bird flu is believed to kill more than half the people it infects, making it much more lethal than common strains of the seasonal virus.

According to the World Health Organization, there have been 573 cases of H5N1 bird flu in humans in 15 countries since 2003, with 58.6 per cent resulting in death.